1. Field Of The Invention
This invention relates to ohmic contacts for semiconductor devices and, in particular, to low resistivity contacts for compound semiconductor devices.
2. History Of The Art
In the fabrication of semiconductor devices, lead conducting wires are nearly always coupled to the semiconductor through an intervening layer of metal forming an ohmic (non-rectifying) contact with the semiconductor. In the typical fabrication of such a contact, the semiconductor to be contacted is doped to a high level of conductivity with a single impurity of one conductivity type (either n-type or p-type) and alloyed with a suitable metal. The metal chosen should (1) be capable of making a good ohmic contact; (2) be an excellent conductor; and (3) be metallurgically compatible with the lead conducting wires. Metals commonly used for ohmic contacts are aluminum, gold, nickel, lead, silver, and chromium.
In many applications, it is important that the ohmic contacts have as little resistivity as is possible. In microwave power transistors, for example, the contact resistance is an important factor limiting the frequency response.
In conventional ohmic contacts wherein an elemental or compound semiconductor is doped with only one impurity, the attainable conductivity is limited by the maximum concentration of that impurity which, in turn, is limited by the impurity solubility and partition coefficients. The use of two different impurities of the same conductivity type in an elemental semiconductor produces no appreciable reduction of resistivity because, in substance, the two different impurities are competing for the same impurity sites.
One approach for overcoming these limiting factors involves doping the semiconductor with two impurities of different respective impurity types. This approach is disclosed in applicant's copending application, Ser. No. 841,553 filed Oct. 11, 1977.
The present invention presents an alternative approach which avoids the necessity of using dopants of opposite conductivity type.